I work at Google on the MapsGL effort, and as part of launching MapsGL we decided to launch a unit testing framework we wrote for our shaders. Shaders can get hugely complicated and bug prone, and don't provide a great way to debug them. GLSL Unit hopes to change that.

Check out our demo application here:
http://glslunit.appspot.com/

Source code is here:
http://code.google.com/p/glsl-unit/